\psset{unit=0.7}
%\psset{lightsrc=30 -10 10,linewidth=0.5\pslinewidth}
\psset{viewpoint=100 30 30 rtp2xyz,Decran=50}
\begin{pspicture}(-7,-4)(7,5)
%\psSolid[object=grille,base=-4 4 -4 4,action=draw]
\psSurface[fillcolor=blue!50,ngrid=.25 .25,incolor=yellow,algebraic,linewidth=0.25\pslinewidth,linecolor=gray](-4,-4)(4,4){x*y/2}
\axesIIID(4,1,3)(6,5,7)
\end{pspicture}